Preambles in OFDMA system

ABSTRACT

The present invention provides a preamble that is inserted into an OFDMA frame and has a common sequence for all the base stations participating in a transmission. The subscriber station performs fine synchronization using the common sequence on the common preamble, and the resulting peaks will provide the locations of candidate base stations. The base station specific search is then performed in the vicinities of those peaks by using base station specific pseudo-noise sequences. With this two stage cell search, the searching window is drastically reduced. The preamble is matched to known values by a respective receiver to decode the signals and permit multiple signals to be transferred from the transmitter to the receiver. The preamble may comprise two parts, Preamble-I and Preamble-2, which may be used in different systems, including multioutput, multi-input (MIMO) systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 11/630,474, filed on Jul. 23, 2007, entitled “PREAMBLES IN OFDMASYSTEM,” which is hereby incorporated herein by reference in itsentirety, and which is a National Phase filing of PCT/CA2005/000987,which claims the benefit of the filing date of U.S. ProvisionalApplication No. 60/582,298, filed on Jun. 24, 2004, and U.S. ProvisionalApplication No. 60/598,660, filed on Aug. 4, 2004, both of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the delivery of data via a wirelessconnection and, more particularly, to the accurate delivery of data athigh rates via a wireless connection.

BACKGROUND OF THE INVENTION

Recent growth in demand for broadband wireless services enables rapiddeployment of innovative, cost-effective, and interoperable multi-vendorbroadband wireless access products, providing alternatives to wire linebroadband access for applications such as telephony, personalcommunications systems (PCS) and high definition television (HDTV). Atthe same time, broadband wireless access has been extended from fixed tomobile subscriber stations, for example at vehicular speed. Though thedemand for these services is growing, the channel bandwidth over whichthe data may be delivered is limited. Therefore, it is desirable todeliver data at high speeds over this limited bandwidth in an efficient,as well as cost effective, manner.

In the ever-continuing effort to increase data rates and capacity ofwireless networks, communication technologies evolve. An encouragingsolution for the next generation broadband wireless access deliveringhigh speed data over a channel is by using Orthogonal Frequency DivisionMultiplexing (OFDM). The high-speed data signals are divided into tensor hundreds of lower speed signals that are transmitted in parallel overrespective frequencies within a radio frequency (RF) signal that areknown as subcarrier frequencies (“subcarriers”). The frequency spectraof the subcarriers may overlap so that the spacing between them isminimized. The subcarriers are also orthogonal to each other so thatthey are statistically independent and do not create crosstalk orotherwise interfere with each other. When all of the allocated spectrumcan be used by all base stations, the channel bandwidth is used muchmore efficiently than in conventional single carrier transmissionschemes such as AM/FM (amplitude or frequency modulation), in which onlyone signal at a time is sent using only one radio frequency, orfrequency division multiplexing (FDM), in which portions of the channelbandwidth are not used so that the subcarrier frequencies are separatedand isolated to avoid inter-carrier interference (ICI).

In OFDM, each block of data is converted into parallel form and mappedinto each subcarrier as frequency domain symbols. To get time domainsignals for transmission, an inverse discrete Fourier transform or itsfast version, IFFT, is applied to the symbols. The symbol duration ismuch longer than the length of the channel impulse response so thatinter-symbol interference is avoided by inserting a cyclic prefix foreach OFDM symbol. Thus, OFDM is much less susceptible to data losscaused by multipath fading than other known techniques for datatransmission. Also, the coding of data onto the OFDM subcarriers takesadvantage of frequency diversity to mitigate loss fromfrequency-selective fading when forward error correction (FEC) isapplied.

Another approach to providing more efficient use of the channelbandwidth is to transmit the data using a base station having multipleantennas and then receive the transmitted data using a remote stationhaving multiple receiving antennas, referred to as MultipleInput-Multiple Output (MIMO). The data may be transmitted such there isspatial diversity between the signals transmitted by the respectiveantennas, thereby increasing the data capacity by increasing the numberof antennas. Alternatively, the data is transmitted such that there istemporal diversity between the signals transmitted by the respectiveantennas, thereby reducing signal fading.

In orthogonal frequency division multiplexing access (OFDMA) systems,multiple users are allowed to transmit simultaneously on the differentsubcarriers per OFDM symbol. In an OFDMA/TDMA embodiment, for example,the OFDM symbols are allocated by time division multiplexing access(TDMA) method in the time domain, and the subcarriers within an OFDMsymbols are divided in frequency domain into subsets of subcarriers,each subset is termed a subchannel.

In OFDMA system, a preamble may be used to provide: base stationidentification and selection, CIR measurement, framing and timingsynchronization, frequency synchronization as well as channelestimation.

Currently preamble is specified only for single antenna transmission,and does not provide a way to efficiently estimate channels frommultiple base station antennas in MIMO environment. The ability tomeasure the channel quality in the entire bandwidth is beneficial, ifeach subscriber station scans the entire frequency bandwidth and selectsthe best band to be used in the subsequent frames. Since the channelsseen from different transmit antennas are more or less uncorrelated,choosing the best band based only on a channel from a single transmitantenna is far from being optimal.

In addition, the current preamble per IEEE 802.16-2004 is designedprimarily for fixed deployment. The preamble search requires a largeamount of computation power at the subscriber station for system accessand for cell selection and reselection to support the device mobility ina multi-cell deployment scenarios and to perform frequency domain finesynchronization. For the initial cell search, there is no priorknowledge about the synchronization positions for potential base stationcandidates; hence the subscriber station needs to perform thecorrelations with all possible pseudo noise (PN) sequences for eachFourier fast transform (FFT) window position within the entire searchingwindow. Such a window could be large even for the synchronous basestation network. For handoff, even with the presence of the adjacentbase station list information broadcast from the anchoring base station,the preamble search is of excessively high computational complexity.

It is therefore desirable to provide preambles enabling easy, fastsynchronization between the subscriber station and the base stations,supporting channel estimation in MIMO environment; being compatible withnon-MIMO subscriber stations; and providing low complexity and fast cellsearch by fine tuning after coarse synchronization.

Accordingly, there is a need for an improved preamble design, method andapparatus which are suitable for the mobile, broadband wireless accesssystems. It is further desirable to provide an improved preamble design,method and apparatus to a MIMO OFDMA system.

SUMMARY OF THE INVENTION

The present invention provides a preamble that is inserted periodicallyinto OFDMA frames and has a common sequence for all the base stationsparticipating in a transmission. The subscriber station performs finesynchronization using the common sequence on the common preamble, andthe resulting peaks will provide the locations of candidate basestations. The base station specific search is then performed in thevicinities of those peaks by using base station specific pseudo-noisesequences. With this two stage cell search, the searching window isdrastically reduced. The preamble is matched to known values by arespective receiver to decode the signals and permit multiple signals tobe transferred from the transmitter to the receiver. The preamble maycomprise two parts, Preamble-1 and Preamble-2, which may be used indifferent systems, including multi-output, multi-input (MIMO) systems.

In accordance with one aspect of the present invention there is provideda method for accessing an orthogonal frequency division multiplexingaccess (OFDMA) system comprising the steps of: a) constructing an OFDMAframe having a training sequence; b) assigning said training sequencefor use in a plurality of base stations, c) transmitting said trainingsequence by said plurality of base stations; and d) detecting saidtraining sequence at said subscriber station; e) performingsynchronization at said subscriber station using said training sequence.

In one embodiment, the training sequence is a first preamble having acommon synchronization channel comprising common synchronizationsubcarriers, said common synchronization subcarriers canes a commonsequence; said common sequence provides locations of candidate basestations, and reduces a search window for base station specificpreamble.

In another embodiment, the training sequence is a second preamble, saidsecond preamble comprising cell-specific synchronization subcarriers,and the method further comprising the step of assigning said preamblefor use in said plurality of base stations and performing cell search atsaid subscriber station.

In accordance with another aspect of the present invention there isprovided an orthogonal frequency division multiplexing access (OFDMA)system comprising: a) a base station controller adapted to schedule databeing transmitted; b) a plurality of base stations operativelyassociated with said base station controller, each base station beingadapted to receive at least a portion of scheduled data from said basestation controller, and to transmit a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols to said subscriber station; saidplurality of base stations transmitting an OFDMA frame having a trainingsequence; and c) a subscriber station using said training sequence toprovide locations of candidate base stations.

In one embodiment, the training sequence is a first preamble having acommon synchronization channel comprising common synchronizationsubcarriers, said common synchronization subcarriers carries a commonsequence, said common sequence provides locations of candidate basestations, and reducing a search window for base station specificpreamble.

In another embodiment, the training sequence is a second preamble, saidsecond preamble comprising cell-specific synchronization subcarriers,said second preamble is assigned for use in said plurality of basestations and performing cell search at said subscriber station.

In accordance with another aspect of the present invention there isprovided a base station in an orthogonal frequency division multiplexingaccess (OFDMA) system comprising: a) a training sequence configuringlogic for generating a training sequence, said training sequence havinga plurality of subcarriers; b) Inverse Fourier Transform (IFT) logicadapted to provide an IFT on each of said subcarriers to generate aplurality of OFDM symbols; and c) transmit circuitry having transmittingsaid plurality of OFDM signals for reception by a subscriber station;said subscriber station performing synchronization using said trainingsequence.

In accordance with another aspect of the present invention there isprovided a subscriber station in an orthogonal frequency divisionmultiplexing access (OFDMA) system comprising: a) receive circuitryadapted to receive and downconvert a plurality of OFDM signals, saidplurality of OFDM symbols forming an OFDMA frame having a trainingsequence; b) Fourier Transform (FT) logic adapted to provide a FT oneach of said plurality of OFDM signals to generate a plurality ofdivided-multiplexed coded signals; and c) decoder logic adapted toprovide division-multiplexing decoding on said plurality ofdivided-multiplexed coded signals to recover data from a plurality ofbase stations.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The invention and the illustrated embodiments may be better understood,and the numerous objects, advantages, and features of the presentinvention and illustrated embodiments will become apparent to thoseskilled in the art by reference to the accompanying drawings. In thedrawings, like reference numerals refer to like parts throughout thevarious views of the non-limiting and non-exhaustive embodiments of thepresent invention, and wherein:

FIG. 1 is a block representation of a cellular communication system;

FIG. 2 is a block representation of a base station according to oneembodiment of the present invention;

FIG. 3 is a block representation of a subscriber station according toone embodiment of the present invention;

FIG. 4 is a logical breakdown of an OFDMA transmitter architectureaccording to one embodiment of the present invention;

FIG. 5 is a logical breakdown of an OFDMA receiver architectureaccording to one embodiment of the present invention;

FIG. 6(a) depicts an example of an OFDM symbol structure in time domain;

FIG. 6(b) shows an example of a basic structure of an OFDMA symbol infrequency domain;

FIG. 7 shows an example of subchannel arranged in frequency domain;

FIG. 8 shows a time plan for the OFDMA frame structure in time divisionduplex (TDD) mode;

FIG. 9(a) shows an example of a cluster;

FIG. 9(b) shows an example of a tile;

FIG. 10(a) illustrates an OFDMA frame with multiple zones;

FIG. 10(b) illustrates an embodiment of the present invention of anOFDMA frame with Preamble-1 and Preamble-2;

FIG. 10(c) illustrates another embodiment of Preamble-1 and Preamble-2in different OFDMA frames;

FIG. 10(d) illustrates another embodiment of Preamble-1 and Preamble-2in an OFDMA frame;

FIG. 10(e) illustrates another embodiment of Preamble-1 and twoPreambles-2 in an OFDMA frame;

FIG. 11(a) shows an embodiment of Preamble-1 in an OFDMA frame, incoexistence with legacy preamble;

FIG. 11(b) shows another embodiment of Preamble-1 at the end of an OFDMAframe;

FIG. 11(c) shows another embodiment of Preamble-1 in an OFDMA frame, ata predefined location;

FIG. 12(a) shows an example of the structure of Preamble-1 in FUSC modein frequency domain;

FIG. 12(b) shows an example of the structure of Preamble-1 in PUSC modein frequency domain;

FIG. 12(c) shows an example of the structure of Preamble-1 in timedomain after IFFT;

FIG. 13(a) shows an example of a primary common synchronization channeland a secondary common synchronization channel in FUSC mode;

FIG. 13(b) shows an example of a primary common synchronization channeland a secondary common synchronization channel in PUSC mode;

FIG. 14 shows an example of Preamble-1 mapped to two antennas is cyclicshift in frequency domain;

FIG. 15(a) depicts an example of Preamble-2 structure for FFT size 1024transmission in FUSC mode;

FIG. 15(b) depicts an example of Preamble-2 structure for FFT sizes 128,256, and 512 transmissions in FUSC mode;

FIG. 15(c) shows an example of a time plan representation of thescalable synchronization performance of the Preamble-2.

FIG. 16(a) depicts an example of Preamble-2 structure for FFT size 1024transmission in PUSC mode at different times t₁ and t₂;

FIG. 16(b) depicts an example of Preamble-2 structure for FFT sizes 128,256, and 512 transmissions in PUSC mode at different times t₁ and t₂;

FIG. 17 shows another example of Preamble-2 using time diversity;

FIG. 18(a) shows an example of Walsh code and antenna mapping in FUSCmode;

FIG. 18(b) shows an example of Walsh code and antenna mapping in PUSCmode;

FIG. 18(c) shows an example of Walsh chip mapping onto subcarriers intime direction;

FIG. 18(d) shows an example of Walsh chip mapping onto subcarriers inboth time and frequency direction;

FIG. 19(a) depicts an example of Steiner approach for cyclic shift intime for Preamble-2 in FUSC mode;

FIG. 19(b) depicts an example of Steiner approach for cyclic shift intime for Preamble-2 in PUSC mode.

FIG. 20(a) is an example of a hybrid FFT size 1024 transmission ofPreamble-2 in FUSC mode;

FIG. 20(b) is an example of a hybrid transmission of Preamble-2 for FFTsizes 128, 256, and 512 transmissions in FUSC mode;

FIG. 20(c) shows an example of relation between the antenna and segmentmapping;

FIG. 21(a) shows an example for search and demodulation of Preamble-1and DL_MAP by multiple antennas;

FIG. 21(b) shows an example of single receive antenna search anddemodulation for Preamble-1 and DL_MAP;

FIG. 22 shows an exemplary flowchart of search and detection of basestations using an embodiment of the present invention; and

FIG. 23 shows another exemplary flowchart of search and detection ofbase stations using an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to some specific embodiments of theinvention including the best modes contemplated by the inventors forcarrying out the invention. Examples of these specific embodiments areillustrated in the accompanying drawings. While the invention isdescribed in conjunction with these specific embodiments, it will beunderstood that it is not intended to limit the invention to thedescribed embodiments. On the contrary, it is intended to coveralternatives, modifications, and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claims.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

In this specification and the appended claims, the singular forms “a,”“an,” and “the” include plural reference unless the context clearlydictates otherwise. Unless defined otherwise, all technical andscientific terms used herein have the same meaning as commonlyunderstood to one of ordinary skill in the art to which this inventionbelongs.

The term “subscriber station” is intended to include any device whichmay provide connectivity between subscriber equipment and a base station(BS). A subscriber station may be fixed, or mobile. When the subscriberstation is mobile, the speed of its mobile carrier should be apparent toa person skilled in the art, for example, the speed of an automobile, anaircraft or a satellite. The term “base station” is intended to includegeneralized equipment set providing connectivity, management, andcontrol of the subscriber station (SS). The term “protocol data unit”(PDU) is intended to describe a data unit exchanged between peerentities of the same protocol layer. The term “service data unit” (SDU)is intended to describe a data unit exchanged between two adjacentprotocol layers.

Referring to FIG. 1, in a wireless communication system 100 a basestation controller (BSC) 102 controls base stations (BS) 104 withincorresponding cells 106. In general, each base station 104 willfacilitate communications with subscriber stations 108, which are withinthe cell 106 associated with the corresponding base station 104. As asubscriber station 108 moves from a first cell 106 a to a second cell106 b, communications with the subscriber station 108 transition fromone base station 104 to another. The term “handoff” is generally used torefer to techniques for switching from one base station 104 to anotherduring a communication session with a subscriber station 108. The basestations 104 cooperate with the base station controller 102 to ensurethat handoffs are properly orchestrated, and that data intended for thesubscriber station 108 is provided to the appropriate base station 104currently supporting communications with the subscriber station 108.

In FIG. 1, a handoff area 110 is illustrated at the junction of threecells 106, wherein a subscriber station 108 b is at the edge of any oneof the three cells 106 and could potentially be supported by any of thebase stations 104 a, 104 b and 104 c within those cells 106 a, 106 b and106 c. The present invention provides a method and architecture forpreambles used in orthogonal frequency division multiplexing access(OFDMA) wireless communication environment. Orthogonal frequencydivision multiplexing access (OFDMA) allows multiple users, for examplesubscriber station 108 a and 108 b, to transmit simultaneously on thedifferent subcarriers per OFDM symbol. The subcarriers within an OFDMsymbols are divided by OFDMA method in frequency domain into subsets ofsubcarriers, which is termed a subchannel. These subchannels are thebasic allocation unit. Each allocation of subchannel may be allocatedfor several OFDM symbols in such a way that the estimation of eachsubchannel is done in frequency and time. The subchannel may be spreadover the entire bandwidth. Therefore, in the OFDMA/TDMA embodiment, OFDMsymbols are shared both in time and in frequency (by subchannelallocation) between different users. As it will be described later, anSHO zone having the same subchannel definition, for example, permutationcode could be defined to facilitate the handoff, to provide RFcombining, to reduce interference; and to provide selection combining.

A high level overview of the subscriber stations 108 and base stations104 of the present invention is provided prior to delving into thestructural and functional details of the preferred embodiments. Withreference to FIG. 2, a base station 104 configured according to oneembodiment of the present invention is illustrated. The base station 104generally includes a control system 202, a baseband processor 204,transmit circuitry 206, receive circuitry 208, multiple antennas 210,and a network interface 212. The receive circuitry 208 receives radiofrequency signals bearing information from one or more remotetransmitters provided by subscriber stations 108 (illustrated in FIG.3). Preferably, a low noise amplifier and a filter (not shown) cooperateto amplify and remove broadband interference from the signal forprocessing. Downconversion and digitization circuitry (not shown) willthen downconvert the filtered, received signal to an intermediate orbaseband frequency signal, which is then digitized into one or moredigital streams.

The baseband processor 204 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 204 is generallyimplemented in one or more digital signal processors (DSPs). Thereceived information is then sent across a wireless network via thenetwork interface 212 or transmitted to another subscriber station 108serviced by the base station 104. The network interface 212 willtypically interact with the base station controller and acircuit-switched network forming a part of a wireless network, which maybe coupled to the public switched telephone network (PSTN) or InternetProtocol (IP) network.

On the transmit side, the baseband processor 204 receives digitizeddata, which may represent voice, data, or control information, from thenetwork interface 212 under the control of control system 202, whichencodes the data for transmission. The encoded data is output to thetransmit circuitry 206, where it is modulated by a carrier signal havinga desired transmit frequency or frequencies. A power amplifier (notshown) will amplify the modulated carrier signal to a level appropriatefor transmission, and deliver the modulated carrier signal to theantennas 210 through a matching network (not shown). Modulation andprocessing details are described in greater detail below.

With reference to FIG. 3, a subscriber station 108 configured accordingto one embodiment of the present invention is illustrated. Similarly tothe base station 104, the subscriber station 108 will include a controlsystem 302, a baseband processor 304, transmit circuitry 306, receivecircuitry 308, multiple antennas 310, and user interface circuitry 312.The receive circuitry 308 receives radio frequency signals bearinginformation from one or more base stations 104. Preferably, a low noiseamplifier and a filter (not shown) cooperate to amplify and removebroadband interference from the signal for processing. Downconversionand digitization circuitry (not shown) will then downconvert thefiltered, received signal to an intermediate or baseband frequencysignal, which is then digitized into one or more digital streams.

The baseband processor 304 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed in greater detail below. Thebaseband processor 304 is generally implemented in one or more digitalsignal processors (DSPs) and application specific integrated circuit(ASIC).

For transmission, the baseband processor 304 receives digitized data,which may represent voice, data, or control information, from thecontrol system 302, which it encodes for transmission. The encoded datais output to the transmit circuitry 305, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). Various modulation and processing techniques available tothose skilled in the art are applicable to the present invention.

In OFDM modulation, the transmission band is divided into multiple,orthogonal carrier waves. Each subcarrier wave is modulated according tothe digital data to be transmitted. Because OFDM divides thetransmission band into multiple carriers, the bandwidth per carrierdecreases and the modulation time per carrier increases. Since themultiple carriers are transmitted in parallel, the transmission rate forthe digital data, or symbols, on any given carrier is lower than when asingle carrier is used.

OFDM modulation requires the performance of an Inverse Fast FourierTransform (IFFT) on the information to be transmitted. For demodulation,the performance of a Fast Fourier Transform (FFT) on the received signalis required to recover the transmitted information. In practice, theInverse Discrete Fourier Transform (IDFT) and Discrete Fourier Transform(DFT) may be implemented using digital signal processing for modulationand demodulation, respectively.

Accordingly, the characterizing feature of OFDM modulation is thatorthogonal carrier waves are generated for multiple bands within atransmission channel. The modulated signals are digital signals having arelatively low transmission rate and capable of staying within theirrespective bands. The individual carrier waves are not modulateddirectly by the digital signals. Instead, all carrier waves aremodulated at once by IFFT processing.

With reference to FIG. 4, a logical OFDM transmission architecture isprovided according to one embodiment. Initially, data 402 to betransmitted to a subscriber station 108 is received at the base station104. The data is scrambled in a manner reducing the peak-to-averagepower ratio associated with the data using data scrambling logic 404. Acyclic redundancy check (CRC) for the scrambled data is determined andappended to the scrambled data using CRC logic 406. Next, channel codingis performed using channel encoder logic 408 to effectively addredundancy to the data to facilitate recovery and error correction atthe subscriber station 108. The channel encoder logic 408 may useforward error correction techniques such as ConcatenatedReed-Solomon-convolutional code (RS-CC), block turbo coding (BTC) orconvolutional turbo codes (CTC). The encoded data is then processed byrate matching logic 410 to compensate for the data expansion associatedwith encoding.

Bit interleaver logic 412 systematically reorders the bits in theencoded data to ensure that adjacent coded bits are mapped ontononadjacent subcarriers, thereby minimize the loss of consecutive databits. This is considered the first step of a two step permutation. Allencoded data bits shall be interleaved by a block interleaver with ablock size corresponding to the number of coded bits per allocatedsubchannels per OFDM symbol. The second step ensures that adjacent codedbits are mapped alternately onto less or more significant bits of theconstellation, thus avoiding long runs of lowly reliable bits.

The resultant data bits are mapped into corresponding symbols dependingon the chosen baseband modulation by mapping logic 414. Binary PhaseShift Key (BPSK), Quadrature Amplitude Modulation (QAM), for example,16-QAM and 64-QAM, or Quadrature Phase Shift Key (QPSK), for example,Gray mapped QPSK modulation may be used. When QAM is used, thesubchannels are mapped onto corresponding complex-valued points in a2^(m)-ary constellation. A corresponding complex-valued 2^(m)-ary QAMsub-symbol, c_(k)=a_(k)+jb_(k), that represent a discrete value of phaseand amplitude, where −N≤k≤N, is assigned to represent each of thesub-segments such that a sequence of frequency-domain sub-symbols isgenerated.

Each of the complex-valued, frequency-domain sub-symbols c_(k) is usedto modulate the phase and amplitude of a corresponding one of 2N+1subcarrier frequencies over a symbol interval T_(s).

The modulated subcarriers are each modulated according to a sinc x=(sinx)/x function in the frequency domain, with a spacing of 1/T_(s) betweenthe primary peaks of the subcarriers, so that the primary peak of arespective subcarrier coincides with a null the adjacent subcarriers.Thus, the modulated subcarriers are orthogonal to one another thoughtheir spectra overlap.

The symbols may be systematically reordered to further bolster theimmunity of the transmitted signal to periodic data loss caused byfrequency selective fading using symbol interleaver logic 416. For thispurpose, specific Reed-Solomon permutation may be used to make thesubchannels as independent as possible from each other. The independenceof the subchannel allocation gives maximum robustness and statisticallyspreading interference between neighboring cells as well as neighboringcarriers between two channels and statistically spreading theinterference inside the cell.

At this point, groups of bits have been mapped into symbols representinglocations in an amplitude and phase constellation. The STC encoder logic418 will process the incoming symbols and provide n outputscorresponding to the number of transmit antennas 210 for the basestation 104. The control system 202 and/or baseband processor 204 willprovide a mapping control signal to control STC encoding. At this point,assume the symbols for the n outputs are representative of the data tobe transmitted and capable of being recovered by the subscriber station108.

For the present example, assume the base station 104 has two antennas210 (n=2) and the STC encoder logic 418 provides two output streams ofsymbols. Accordingly, each of the symbol streams output by the STCencoder logic 418 is sent to a corresponding IFFT processor 420,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch digital signal processing alone or in combination with otherprocessing described herein. The IFFT processors 420 will preferablyoperate on the respective symbols using IDFT or like processing toeffect an inverse Fourier Transform. The output of the IFFT processors420 provides symbols in the time domain.

It should be apparent to a person skilled in the art that the STCencoder may be a space time transmit diversity (STTD) encoder or aspatial multiplexing (SM) encoder employing, for example, Bell LabsLayered Space-Time (BLAST).

The time domain symbols are grouped into frames, which are associatedwith prefix and pilot headers by like insertion logic 422. Each of theresultant signals is up-converted in the digital domain to anintermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUC) and digital-to-analog (D/A)conversion circuitry 424. The resultant (analog) signals are thensimultaneously modulated at the desired RF frequency, amplified, andtransmitted via the RF circuitry 426 and antennas 210. Notably, thetransmitted data is preceded by pilot signals, which are known by theintended subscriber station 108 and implemented by modulating the pilotheader and scattered pilot subcarriers. The subscriber station 108,which is discussed in detail below, will use the-scattered pilot signalsfor channel estimation and interference suppression and the header foridentification of the base station 104.

Reference is now made to FIG. 5 to illustrate reception of thetransmitted signals by a subscriber station 108. Upon arrival of thetransmitted signals at each of the antennas 310 of the subscriberstation 108, the respective signals are demodulated and amplified bycorresponding RF circuitry 502. For the sake of conciseness and clarity,only one of the two receive paths is described and illustrated indetail. Analog-to-digital (A/D) converter and down-conversion circuitry504 digitizes and downconverts the analog signal for digital processing.The resultant digitized signal may be used by automatic gain controlcircuitry (AGC) 506 to control the gain of the amplifiers in the RFcircuitry 502 based on the received signal level.

Preferably, each transmitted frame has a defined structure having twoidentical headers. Framing acquisition is based on the repetition ofthese identical headers. Initially, the digitized signal is provided tosynchronization logic 508, which includes coarse synchronization logic510, which buffers several OFDM symbols and calculates anauto-correlation between the two successive OFDM symbols. A resultanttime index corresponding to the maximum of the correlation resultdetermines a fine synchronization search window, which is used by thefine synchronization logic 512 to determine a precise framing startingposition based on the headers. The output of the fine synchronizationlogic 512 facilitates frequency acquisition by the frequency alignmentlogic 514. Proper frequency alignment is important so that subsequentFFT processing provides an accurate conversion from the time to thefrequency domain. The fine synchronization algorithm is based on thecorrelation between the received pilot signals carried by the headersand a local copy of the known pilot data. Once frequency alignmentacquisition occurs, the prefix of the OFDM symbol is removed with prefixremoval logic 516 and a resultant samples are sent to frequency offsetand Doppler correction logic 518, which compensates for the systemfrequency offset caused by the unmatched local oscillators in thetransmitter and the receiver and Doppler effects imposed on thetransmitted signals. Preferably, the synchronization logic 508 includesfrequency offset, Doppler, and clock estimation logic 520, which isbased on the headers to help estimate such effects on the transmittedsignal and provide those estimations to the correction logic 518 toproperly process OFDM symbols.

At this point, the OFDM symbols in the time domain are ready forconversion to the frequency domain using the FFT processing logic 522.The results are frequency domain symbols, which are sent to processinglogic 524. The processing logic 524 extracts the scattered pilot signalusing scattered pilot extraction logic 526, determines a channelestimate based on the extracted pilot signal using channel estimationlogic 528, and provides channel responses for all subcarriers usingchannel reconstruction logic 530. The frequency domain symbols andchannel reconstruction information for each receive path are provided toan STC decoder 532, which provides STC decoding on both received pathsto recover the transmitted symbols. The channel reconstructioninformation provides the STC decoder 532 sufficient information toprocess the respective frequency domain symbols to remove the effects ofthe transmission channel.

The recovered symbols are placed back in order using the symbolde-interleaver logic 534, which corresponds to the symbol interleaverlogic 416 of the transmitter. The de-interleaved symbols are thendemodulated or de-mapped to a corresponding bitstream using de-mappinglogic 536. The bits are then de-interleaved using bit de-interleaverlogic 538, which corresponds to the bit interleaver logic 412of thetransmitter architecture. The de-interleaved bits are then processed byrate de-matching logic 540 and presented to channel decoder logic 542 torecover the initially scrambled data and the CRC checksum. Accordingly,CRC logic 544 removes the CRC checksum, checks the scrambled data intraditional fashion, and provides it to the de-scrambling logic 546 forde-scrambling using the known base station de-scrambling code to recoverthe originally transmitted data.

Orthogonal frequency division multiplexing access (OFDMA) allowsmultiple users to transmit simultaneously on the different subcarriersper OFDM symbol. In an OFDMA/TDMA embodiment, the OFDM symbols areallocated by TDMA method in the time domain, and the subcarriers withinan OFDM symbols are divided by OFDMA method in frequency domain intosubsets of subcarriers, each subset is termed a subchannel. Thesubcarriers forming one subchannel may, but need not be adjacent. Thesesubchannels are the basic allocation unit. Each allocation of subchannelmay be allocated for several OFDM symbols in such a way that theestimation of each subchannel is done in frequency and time. Thesubchannel may be spread over the entire bandwidth. This scheme achievesimproved frequency diversity and channel usage without the need forfrequency separation between subcarriers. The allocation of carriers tosubchannel may be accomplished by special Reed-Solomon series, whichenables the optimization and dispersion of interfering signals inside acell and between adjacent cells. Therefore, in the OFDMA/TDMAembodiment, OFDM symbols are shared both in time and in frequency (bysubchannel allocation) between different users. When the OFDMA is usedin the uplink (UL), it allows users to operate with smaller poweramplifiers, at expense of instantaneous data rate. On the other hand itallows allocating dynamically larger amounts of bandwidth to userscapable of utilizing it in terms of the link budget. When applied to thedownlink (DL), OFDMA allows transmitting to multiple users in parallelwith designated data streams, and may improve the link budget ofdisadvantaged users by allocating to their suchannels a larger fractionof their downlink transmit power.

Unless defined otherwise, technical and scientific terms used hereinhave the same meaning as defined in IEEE 806.16-2004 and IEEE 806.16e(available at www.ieee802.org) which are incorporated by reference intheir entireties.

FIG. 6 shows the transmitted OFDM symbols arranged according toincreasing time and increasing subcarrier frequency. The subcarrierfrequencies contained within an OFDM symbol are each represented bycircles. In the time domain, the first two symbols 602 of a frame may bepreamble symbols 610, for example, in the case of a downlink (DL)subframe. Preamble symbols 612 may also be embedded in the frame, forexample, in the case of an uplink (UL) subframe. Data symbols 606 fordata transmission, or scattered pilot symbols for various estimationpurposes 604 are transmitted, depending on the subcarrier frequency,until the next preamble symbols are transmitted. Null subcarriers 608means no transmission, and may be used for guard bands, non-activesubcarriers and the DC subcarrier. The preamble may provide one of thefollowing fundamental operations: fast base station access, base stationidentification/selection and C/I ratio measurement, framing and timingsynchronization, frequency and sampling clock offset estimation andinitial channel estimation. The design of a frame preamble withminimized overhead is critical to maximum spectral efficiency and radiocapacity.

FIG. 6a shows an example of an OFDMA symbol structure in time domain.OFDMA waveform is created by Inverse-Fourier-Transform. The timeduration 602 is referred to as the useful symbol time T_(b). A copy of asegment 704 (last T_(g)) of the useful symbol period 602, termed cyclicprefix (CP), is copied and appended to the beginning of the usefulsymbol time T_(b) 606, and may be used to collect multipath, whilemaintaining the orthogonality of the tones. Using a cyclic extension,the samples required for performing the FFT at the receiver may have asmall range of timing error, compared to the length of prefix, over thelength of the extended symbol. This provides multipath immunity as wellas a tolerance for symbol time synchronization errors.

An OFDMA symbol may be characterized by following primitive parameters:the nominal bandwidth (BW); the number of used subcarriers (N_(used)),for example, 1703; sampling factor n, which in conjunction with BW andN_(used) determines the subcarrier spacing, and the useful symbol time,and the ratio of CP time T_(g) to useful symbol time T_(b) (G), forexample, ¼, ⅛, 1/16 or 1/32.

Based on the primitive parameters, other parameters could be derived:the FTT size N_(FFT) which is the smallest power of two greater thanN_(used), for the above example of N_(used)=1703, N_(FFT) is 2048;sampling frequency F_(s)=floor (n·8/7·BW/8000)×8000; Subcarrier spacing:Δf=F_(s)/N_(FFT); useful symbol time: T_(b=)1/Δf; CP Time:T_(g)=G·T_(b); OFDMA Symbol Time: T_(s)=T_(b)+T_(g); and sampling time:T_(b)/N_(FFT).

Referring to FIG. 6b , a basic structure of an OFDMA symbol 610 isdescribed in frequency domain. As discussed in the above, an OFDMAsymbol is made up of subcarriers 612-618, the number of which generallycorrelates to the FFT size used. There may be several subcarrier types:data subcarriers 612, 616, 618 are used for data transmission; pilotsubcarriers 614 are used for various estimation purposes; and nullcarrier has no transmission at all, for guard bands 620 and DC carrier.Guard bands 620 are used to enable the signal to naturally decay andcreate the FFT “brick wall” shaping. In OFDMA, active subcarriers aredivided into subsets of subcarriers, each subset is termed a subchannel.The symbol is divided into subchannels to support scalability, multipleaccess, and advanced antenna array processing capabilities. In FIG.6(b), three distinct subchannels 612, 616, and 618 are illustrated. Tensand hundreds of subchannels may be implemented. In the downlink, asubchannel may be intended for different (groups of) receivers; in theuplink, a transmitter may be assigned one or more subchannels, severaltransmitters may transmit simultaneously. The subcarriers forming onesubchannel may, but need not be adjacent.

In FIG. 7, each horizontal arrow 702 in the frequency domain 704represents a logical subchannel. The symbol is divided into subchannelsto support scalability, multiple access, and advanced antenna arrayprocessing capabilities. A minimum number of symbols are allocated toone subchannel, this may be accomplished by special Reed-Solomon series,which enable the optimization and dispersion of interfering signalsinside a cell and between adjacent cells. Each subchannel is the basicallocation unit that a user can be allocated. In the time domain 706,OFDM symbols 708 are shown in columns as FIG. 7.

When in a time plan such as the one illustrated in FIG. 8, a slot 802 isdefined as a pair of an OFDM time symbol number and a subchannel logicalnumber. A subchannel is the minimum possible data allocation unit, andits size may vary for uplink and downlink, for full usedsubchannelization (FUSC) and partially used subchannelization (PUSC),and for the distributed subcarrier permutations and the adjacentsubcarrier permutation, between one subchannel by one OFDMA symbol toone subchannel by three OFDMA symbols. For example, in DL and UL PUSCwhich will be discussed below, the DL and UL subframe size and thegranularity of the DL and UL allocations are one by two or one by threeOFDM symbols, respectively. These slots may be referred to as clustersor tiles composing two and three OFDM symbols, respectively.

In OFDMA, a data region is a two-dimensional allocation of a group ofcontiguous subchannels, in a group of contiguous OFDMA symbols. Examplesof data regions are shown in FIG. 8.

The DL-MAP message, if transmitted in the current frame, is the firstMAC PDU in the burst following the FCH. An UL-MAP message followsimmediately either the DL-MAP message (if one is transmitted) or theDLFP. If Uplink Channel Descriptor (UCD) and Downlink Channel Descriptor(DCD) messages are transmitted in the frame, they follow immediately theDL-MAP and UL-MAP messages.

Simultaneous DL allocations can be broadcast, multicast, and unicast andthey can also include an allocation for another base station rather thana serving base station. Simultaneous ULs can be data allocations andranging or bandwidth requests.

There are two major subchannel allocation methods in the downlink:partial usage of subchannels (PUSC) where some of the subchannels areallocated to the transmitter, and full usage of the subchannels (FUSC)where all subchannels are allocated to the transmitter. In FUSC, thereis one set of common pilot subcarriers, but in PUSC, each subchannelcontains its own set of pilot subcarriers.

For FUSC in the downlink 810, the pilot tones are allocated first; thenthe zero carriers, then all the remaining subcarriers are used as datasubcarriers, which are divided into subchannels that are usedexclusively for data. There are two variable pilot-sets and two constantpilot-sets. In FUSC, each segment uses both sets of variable/constantpilot-sets.

Assuming an FTT size of 2048 is used, each subchannel in FUSC maycomprise 48 subcarriers. The subchannel indices may be formulated usinga Reed-Solomon series, and is allocated out of the data subcarriersdomain. The data subcarriers domain includes 48*32=1536 subcarriers,which are the remaining subcarriers after removing from the subcarrier'sdomain (0-2047), the variable set and the constant set of pilots, guardsubcarriers and the DC subcarrier.

The 1536 data subcarriers are partitioned into groups of contiguoussubcarriers. Each subchannel consists of one subcarrier from each ofthese groups. The number of groups is therefore equal to the number ofsubcarriers per subchannel, N_(subcarrier). The number of thesubcarriers in a group is equal to the number of subchannels,N_(subchannels). The partitioning of subcarriers into subchannels can beexpressed in the following permutation formula.subcarrier (k,s)=N _(subchannels) ·n _(k)+{p _(s) [n _(k) mod N_(subchannels) ]+IDcell} mod N _(subchannels)

Wherein subcarrier (k,s) is the subcarrier index of subcarrier n insubchannel s, s is the index number of a subchannel, from the set [0 . .. N_(subchannels)−1], n_(k)=(k+13·s) mod N_(subchannels), where k is thesubcarrier-in-subchannel index from the set [0 . . . N_(subchannels)−1],N_(subchannels) is the number of subchannels, p_(s)[j] is the seriesobtained by rotating {PermutationBase₀} cyclically to the left s times,ceil[ ] is the function that rounds its argument up to the next integer,IDcell is an integer ranging from 0 to 31, which identifies theparticular base station segment and is specified by MAC layer, andX_(mod(k)) is the remainder of the quotient X/k (which is at most k−1).

For PUSC in the downlink or in the uplink 810, the set of usedsubcarriers is first partitioned into subchannels, and then the pilotsubcarriers are allocated from within each subchannel.

In a downlink using PUSC, a symbol is first divided into basic clustersas illustrated in FIG. 9(a). Pilots 906 and data carriers 908 areallocated within each cluster 902 904. For an OADM symbol of FFT size2048, the number of used subcarriers, after subtracting the guardsubcarriers (367), is 1681. Each cluster may have 14 subcarriers for atotal of 120 clusters. For the 60 subchannels the allocation ofsubcarriers is as following:

1) Dividing the subcarriers into 120 physical clusters containing 14adjunct subcarriers each (starting from carrier 0).

2) Renumbering the physical clusters into logical clusters using thefollowing formula (As illustrated in FIG. 8, the first PUSC zone of thedownlink, the default IDcell is 0):LogicalCluster=RenumberingSequence((PhysicalCluster+13*IDcell) mod 120)

3) Dividing the clusters into six major groups.

4) Allocating carriers to subchannel in each major group is performed byfirst allocating the pilot carriers within each cluster, and then takingall remaining data carriers within the symbol and using the sameprocedure as described above.

Referring to FIG. 8, an uplink 812 using PUSC, following a downlink 810may also support up to three segments. For an OFDM symbol with FFT size2048,a burst in the uplink may be composed of three time symbols and onesubchannel, the three time symbols and one subchannel is termed a tile.Within each burst, there are 48 data subcarriers and 24 fixed-locationpilot subcarrier, a total of 70 subchannels may be supported. Thesubchannel is constructed from six uplink tiles, each tile has foursubcarriers per symbol. FIG. 9(b) shows the structure of a tile withdata subcarrier 912 and pilot subcarrier 910.

The permutation PUSC in UL is based on the allocation of tiles tosubchannels through following steps:

1) Divide the 420 tiles into six groups, containing 70 adjacent tileseach.

2. Choose six tiles per subchannel based onTile(s, n)=70·n+(Pt [(s+n) mod 70]+UL_IDcell) mod 70

wherein n is the tile index 0 . . . 5, Pt is the tile permutation, s isthe subchannel number, UL_IDcell is an integer value in the range 0 . .. 69, which is set by the MAC layer.

After allocating the tiles for each subchannel the data subcarriers persubchannel are allocated as follows:

1) After allocating the pilot carriers within each tile, indexing thedata subcarriers within the subchannels is performed starting from thefirst symbol at the lowest subcarrier from the lowest tile andcontinuing in an ascending manner throughout the subcarriers in the samesymbol, then going to next symbol at the lowest data subcarrier, and soon. Data subcarriers shall be indexed from 0 to 47.

2) The allocation of the subcarriers is as follows:subcarrier(n, s)=(n+13·s)mod N_(subcarriers)

wherein n is a running index 0 . . . 47, s is the subchannel number,N_(subcarriers) is the number of subcarriers per subchannel.

There are two main types of subcarrier permutations: distributed andadjacent. In general, distributed subcarrier permutations perform wellin mobile applications while adjacent subcarrier permutations can beproperly used for fixed, portable, or low mobility environments.

OFDMA DL and UL subframes start in DL and UL PUSC mode, respectively. InDL PUSC, subchannels may be divided and assigned to three segments thatcan be allocated to sectors of the same cell. A sector of a cell may beportioned through means known to a person skilled in the art, forexample, through directional beam.

The available OFDMA subchannels may be divided into subset for deployinga single instance of the MAC, the subset is called a segment. A segmentmay include all available subchannels. In PUSC, for example, any segmenthas at least 12 subchannels. Therefore, a downlink may be divided into athree segments and a preamble structure which begins the transmission.The preamble subcarriers at the beginning of downlink may be alsodivided into three carrier-sets, each of them may be used by one of thesegments in the following manner: segment 0 uses preamble carrier-set 0;segment 1 uses preamble carrier-set 1; and segment 2 uses preamblecarrier-set 2.

Permutation zone is a number of contiguous OFDMA symbols, in the DL orthe UL, that use the same permutation formula. The DL subframe or the ULsubframe may contain more than one permutation zone. An OFDMA frame mayinclude multiple zones as illustrated in FIG. 10. Although the zones inFIG. 10 are shown as vertical columns spanning all the subchannellogical numbers, it should be apparent to a person skilled in the artthat a permutation zone may also have other irregular shapes on a TDDtime plan such as the one illustrated in FIG. 8.

FIG. 10(a) illustrates zone switching within the DL and UL subframes.The switching is performed using an information element included inDL-MAP and UL-MAP. DL and UL subframes both start in PUSC mode wheregroups of subchannels are assigned to different segments by the use ofdedicated FCH messages. The PUSC subcarrier allocation zone 1004 can beswitched to a different type of subcarrier allocation zone through adirective from the PUSC DL-MAP 1004. FIG. 10(a) shows the zone switchingfrom the perspective of a PUSC segment. In FIG. 10(a), the first zonePUSC contains FCH and DL-MAP 1004 is followed with another possibly dataPUSC zone with a parameter “DL_PermBase X” 1006. A FUSC zone for anothersector/cell with “DL_PermBase Y” 1008 is allocated next, followed by anFUSC zone for “DL_PermBase Z” 1010. A switching to FUSC “DL_PermBase M”1012 can then be planned. Optional PUSC, FUSC 922, and AMC 924 zones inDL subframes and optional PUSC 1020 and AMC zones in UL subframes can besimilarly scheduled. Allocation of AMC zones support simultaneouslyfixed, portable, and nomadic mobility users along with high mobilityusers.

The timing synchronization, namely the process whereby the starting andending points of the transmitted data are determined, is usually carriedout in two stages in an OFDM system.

In case each transmitted frame has a defined structure having twoidentical headers, framing acquisition is based on the identification ofthese identical headers. During the first coarse synchronization stage,several OFDM symbols are buffered and an auto-correlation between thetwo successive OFDM symbols is calculated. A resultant time indexcorresponding to the maximum of the correlation result may declare thefirst stage completed.

The fine synchronization process uses one or more preambles stored in amemory, and matches the stored preamble(s) to the transmittedpreamble(s). From the starting point determined by the coarsesynchronization and traversing forward and/or backward along the datastream, the encountered preambles are compared to the stored preamble bycomputing a correlation until a complete match is found.

Pseudo noise (PN) pilots in the preamble are typically used to conductfine synchronization, in either the time domain or in the frequencydomain. For MIMO, the final synchronization position is determined basedon the correlation results from all MIMO channels.

Prior art preamble in OFDMA in MIMO systems is transmitted from a singleantenna, and has no common synchronization channel. The other antennassimply transmit pilots. The pilots for channel estimation for theseother antennas are more sparse and thus not adequate. This is especiallytrue in fast fading channels. In slow fading channels, it is possible tocombine these pilots which are spread across multiple symbols to form asingle channel estimate. However, in fast fading channels (i.e. in highDoppler situations), one may not be able to combine the pilots spreadacross multiple symbols to form a channel estimate since the channelchanges significantly during these symbols. The pilots are more sparsewhen the number of antennas is 3 or 4. It is then important to have agood channel estimate.

More generally, due to the increase of channel bandwidth in broadbandwireless access, along with the increase of FTT size, preamble searchrequires excessively high computational.

In accordance with one embodiment of the present invention, there isprovided a new set of preambles, which may be used for coarse timing andframe synchronization; IDcell identification; indication of the segmentid; frequency synchronization; MIMO channel estimation; FUSC CIRmeasurement and PUSC CIR measurement. The new set of preambles mayfurther support frequency domain fine frequency synchronization.

The new set of preamble functions as a training sequence for asubscriber station to gain access to a base station, or to a pluralityof base stations. Each member of the new set of preambles may coexistwith the existing legacy preamble, or replace the legacy preamble. Theterm “legacy preamble” is intended to include the prior art preamble inOFDMA frame, for example, described in IEEE 802.16-2004.

One preamble of the new set is defined as a common preamble. The commonpreamble uses a common pseudo noise (PN) sequence for all base stations.The term common sequence is intended to indicate the common PN sequenceused to modulate common channel subcarriers. The common channelsubcarriers 1204, 1206, 1212 may carry PN code, e.g. PN_(c)(1),PN_(c)(2) . . . PN_(c)(n). The subscriber station performs finesynchronization using the common PN sequence on the common preamble, andthe resulting peaks will provide the locations of candidate basestations. The base station specific search is then performed in thevicinities of those peaks by using base station specific PN sequences.With this two stage cell search, the searching window is drasticallyreduced.

For the synchronized base station deployment, if the anchor base stationbroadcasts the neighbor base station list for M sectors, the number ofthe cell specific sequences applied in cell search may be reduced. Thecorrelation of the common preamble allows the base station specificpreamble search window to reduce, for example to about 5 samples orless, the common preamble assisted cell specific preamble searching canspeed up the preamble search time by 60 times, or to reduce the searchcomputational complexity by 60 times. The term “Preamble-1 ” is intendedto describe the common preamble in the following description.

Another member of the new set is defined as a cell-specific preamble,comprising cell-specific synchronization subcarriers, it may be used inMIMO or non-MIMO systems. The cell-specific preamble may be termed as“Preamble-2” in the following description.

The allocation and the structure of the two preambles in the newpreamble set, Preamble-1 and Preamble-2, which act as a trainingsequence for the subscriber in facilitating the access to the basestations, will now be described.

FIG. 10(b) illustrates a first embodiment of the present inventionwherein two preambles, Preamble-1, or a legacy preamble, 1022 andPreamble-2 1024 are located in the consecutive OFDMA ZONES, before theFCH and DL_MAP 1026 and a non-MIMO zone 1028. As will be discussedlater, in accordance with one embodiment of the present invention,Preamble-1 may be responsible for coarse synchronization, finding thecandidate fine synchronization positions, indicating the configurationof Preamble-2, and indicating the segment id; while Preamble-2 may beresponsible for IDcell identification; fine synchronization; MIMOchannel estimation; FUSC CIR measurement. PUSC CIR measurement may beextracted from DL_MAP by decision feedback.

FIG. 10(c) illustrates a second embodiment of the present inventionwhere the Preamble-1, (or legacy preamble) 1022 and Preamble-2 1024 areinserted every L frames. If the L is equal to 1, Preamble-1 (or legacypreamble) and Preamble-2 are inserted into alternating frames.

FIG. 10(d) illustrates a third embodiment of the present invention wherethe Preamble-2 1024 is inserted after DL_MAP symbols and before eitherMIMO zone or non-MIMO zone.

If a non-MIMO permutation zone and a MIMO zone are used in the sameframe, the fourth embodiment of the present invention is shown in FIG.10(e). Here, two Preambles-2 1024 are inserted before a non-MIMOpermutation zone 1028 and a MIMO permutation zone 1030, respectively.

Referring to FIG. 11(a), Preamble-1 1104 is at a location indicated byan information element, for example, the DL_Common_SYNC_Symbol_IE in theDL_MAP_IE. Frame L has legacy preamble 1102, non-MIND permutation zone1028 and MIMO permutation zone 1032. Preamble-1 may also be indicated indownlink channel descriptor (DCD) by the TLV values of common synchpreamble transmission cycle, offset, and symbol offset. Preambles mayalso be inserted at the end of the frame as illustrated in FIG. 11(b),or at other predetermined locations, as illustrated in FIG. 11(c), andco-exists with legacy preamble. The predetermined location asexemplified in FIG. 11(c) does not require an indication, for example,in an information element.

Now turning to the structures of the preambles of the present invention,FIG. 12 illustrates an embodiment of a frequency domain Preamble-1structure 1202 applied by all sectors. For FUSC, as shown in FIG. 12(a),Preamble-1 1202 has common synchronization channel consists of commonsynchronization subcarriers 1204, 1206 as every 2nd subcarrier, thenull-subcarriers 1208 are considered as non-transmitting by all sectors.The common PN sequence may have a low Peak to Average Power Ratio(PAPR). FIG. 12(b) is an example of the frequency domain Preamble-1structure 1210 for PUSC where every 6th subcarrier is a common channelsubcarrier 1212. FIG. 12(c) shows an example of an OFDM symbol in FUSC,in time domain after invert Fourier transform of the frequency structuredescribed in FIG. 12(a), with a prefix 1214 and two repeated sequences1216. The term common synchronization channel is intended to indicatethe sequential order of the common channel subcarriers being transmittedby the base stations to facilitate the coarse synchronization. Thecommon channel subcarriers 1204, 1206, 1212 may carry common PN code,e.g. PN_(c)(1), PN_(c)(2) . . . PN_(c)(n).

The repeated time domain structure of Preamble-1 may be used to supportframe synchronization, coarse timing and frequency synchronization byusing auto-correlation peak to identify coarse frame boundary; toprovide the candidate fine synchronization positions for cellidentification by using the cross correlation peak to identify framelocation and to support FFT size identification by usingauto-correlation with different window size (equal to FFT size) toidentify the FFT size, and by auto-correlation to estimate frequencyoffset.

As indicated in Table 1, the number of the OFDM symbols varies fordifferent bandwidth with different FFT sizes to maintain the samesynchronization performance. For example, while in FUSC mode, for FFTsize of 256, the length of the common sequence is 64 while for FFT sizeof 2048, the length of the common sequence is 1024. Accordingly, thenumber of OFDM symbols is 8 for FFT size 128 and 0.5 for FFT size 2048.

TABLE 1 FFT 128 256 512 1024 2048 Sequence length (FUSC) 64 128 256 5121024 Sequence length (PUSC) ~21 ~42 ~85 ~170 ~341 # OFDM symbol forPreamble-1 8 4 2 1 0.5

This advantageous same synchronization performance for different channelbandwidth is maintained through the scalability of the sequence length.However, it is also possible to use same number of OFDM symbols, ordifferent sequence length in different channel bandwidths, atcorresponding varying synchronization performance.

In another embodiment of the present invention, a plurality of commonsequences may be implemented in Preamble-1 to carry other signalinginformation, for example, the number of antennas used in the Preamble-2,or the operation mode of the Preamble-2: PUSC or FUSC. FIG. 13illustrates an example of two groups of subcarriers assigned to carrythe primary common channel 1302 and the secondary common channel 1308.Referring to FIG. 13(a) where Preamble-1 in FUSC mode is shown, thefirst group is mapped onto primary common channel 1304, the second groupis mapped onto secondary common channel 1310. As illustrated, theprimary common channel may use the same sequence 1302 for all sectors,while the secondary common channel may carry multiple sequences 1308,each sequence index is used for carrying signaling information. Forexample, 8 sequences may be used to carry 3 bits signaling information:bit 0 to indicate the operation mode. Preamble-2, bit 1-2 to indicatethe number of antennas. Another example for PUSC mode is shown in FIG.13(b), where the primary common channel subcarriers 1304 and secondarycommon channel subcarriers 1306 are present at every 6 subcarriers.

Preamble-1 may also be located at a pre-determined location, forexample, at the end of the frame, illustrated in FIG. 11(b), or with aflexible location in the frame and indicated by signaling, for example,in DL_MAP as shown in FIG. 11(a).

Preamble-1 and Preamble-2 may be used for different input outputconfigurations. For single-input, single-output (SISO) Preamble-1 istransmitted by single antenna. Four transmission formats formultiple-input, single-output (MISO) or multiple input, multiple output(MIMO) may be used: transmission by single antenna; each antennatransmits the same construction of the Preamble-1; each antennatransmits the same construction of the Preamble-1 but cyclic shift infrequency domain is used; and as shown in FIG. 14(a), same constructionof the Preamble-1, but each of the two antennas is cyclic shift infrequency domain. Referring to FIG. 14(b), Preamble-1 is transmitted onfour separate MIMO antennas with cyclic shift in frequency domain. Itshould be apparent to a person skilled in the art that similararrangements can be easily made for a three antennas or, in general, forN antennas arrangement where N>1. Preamble-1 may also be transmitted on,as shown in FIG. 14(c) on three or four antennas in a combination ofcyclic shift in frequency domain and RF combining. FIG. 14(d) shows anexample of Preamble-1 transmitted on four antennas, in a format of RFcombining.

A second preamble, Preamble-2, may complement the functions ofPreamble-1. Preamble-1 and Preamble-2 may be present in different framesas illustrated in FIG. 10(c), or in the same frame as illustrated inFIGS. 10(d) and (e).

Preamble-2 may be used for sector identification. This sectoridentification may have the following functionalities: cellidentification and selection, fine synchronization, MIMO channelestimation and CIR estimation, which allows the estimation of power fromother sector powers and the channel response. Furthermore, cell specific(IDcell) sequences with low PAPR may be mapped to antenna specificsubcarriers set.

As illustrated in FIGS. 10(d) and (e), Preamble-2 may also be located ina flexible location in the frame and indicated by signaling, forexample, in DL_MAP.

In MIMO transmission, the orthogonality of Preamble-2 is maintained.Referring to FIG. 15(a) where an FFT size 1024 transmission in FUSC modeis illustrated, the subcarriers of Preamble-2 are partitioned into fourgroups 1502, 1504, 1506, and 1508, each antenna modulates the subcarrierin the corresponding group. In case there is only one transmit antennagroup 1502, 1504, 1506, and 1508 are used by single antenna; for twotransmit antennas case, groups 1502 and 1506 are used by one antenna andgroups 1504 and 1508 are used by another antenna; for three transmitantennas case, groups 1502, 1504, and 1506 are used by differentantennas and no-transmission for group 1208; and for four transmitantennas, all four groups 1502, 1504, 1506 and 1508 are used bydifferent antennas.

Here, the advantage of Preamble-2 is demonstrated through theflexibility of the mapping of antennas. The number of the groups isdefined, regardless of the number of antennas. The assignment of theantennas is decided based on the number of antennas used.

These antenna mapping schemes can also be applied to PUSC where only thesubcarriers in the clusters of assigned segment are used by each sector.

As discussed above, to maintain the same synchronization performance fordifferent bandwidth with different FFT sizes, the OFDM symbol numbersneed to be scaled accordingly for corresponding lower FFT sizetransmissions. FIG. 15(b) shows the scaled OFDM symbols for 512-FTT1510, 256-FFT 1512 and 128-FFT 1514 of the corresponding 1024-FFT inFIG. 15(a).

FIG. 15(c) is an example of a time plan representation of the scalablesynchronization performance of the Preamble-2. Preamble-2 is positionedbetween two zones 1516, and 1518. Referring to FIGS. 10(b), (d) and (e),zone 1516 may be Preamble 1, DL_MAP or PUSC/FUSC; zone 1518 may beDL_MAP, MIMO or non-MIMO PUSC/FUSC. For FFT size 1024, Preamble-2 1520has one unit size, for example, one OFDM symbol; for FFT size 512,Preamble-2 1522 has two-unit size; for FFT size 256; the size ofPreamble-2 1524 doubles again; and Preamble-2 1526 for FFT size 128 iseight times of Preamble-2 for 1024-FFT.

Both training sequences, namely Preamble-1 or Preamble-2 may havedifferent sequence for different bandwidths with corresponding differentFFT sizes. Preamble-1 or Preamble-2 may also have a parent sequence forone particular bandwidth/FFT size, and obtain the other sequences forother bandwidths with corresponding FFT sizes through truncation orconcatenation.

FIGS. 13, 14 and 15 illustrate Preamble-1 and Preamble-2 in both FUSCmode and PUSC mode. However, it should be apparent to a person skilledin the art that the examples described here can easily adapted to otherusage modes for allocating the subcarriers. In fact, the flexibility ofthe subcarrier allocation is one of the advantages of the trainingsequences, Preamble-1 and Preamble-2, as described.

FIG. 16 shows another example of Preamble-2 using time division formultiple antenna mapping. FIG. 16(a) show the partition of the usedsubcarriers into two groups 1602, 1604 at different times t₁ and t₂,each antenna modulates the subcarrier in the corresponding group andtime. For example, for one transmit antenna, group 1602 at time t₁ istransmitted; for two transmit antennas group 1602 and 1604 are sent attime t₁; for three transmit antennas transmission groups 1602, 1606 areactive at t₁ and t₂, and group 1608 is active at t₂; and for the fourtransmit antennas case, all four groups 1602, 1604, 1606 and 1608 aretransmitting at t₁ and t₂.

FIG. 16(b) shows the scaled OFDM symbols 1610 for 512-FTT 1612, 256-FFT1614 and 128-FFT 1616 of the corresponding 1024-FFT in FIG. 16(a). Thesub-carrier 1610 represent subcarriers 1602, 1604, 1606 and 1608 at t₁and t₂.

In practice, the power on the modulated tones may be boosted to reachmaximum transmit power allowed.

FIG. 17 is another example of Preamble-2 using time division formultiple antenna mapping. The used subcarriers are partitioned into 6groups, each antenna in each segment modulates the subcarrier in thecorresponding group and time. In PUSC, subcarriers may be divided andassigned to three segments that can be allocated to sectors of the samecell. At t₁, groups 1702 and 1704 are transmitted, and at t₂ groups 1706and 1708 are transmitted, respectively, in each of the three segments1710, 1712, and 1714.

In another embodiment of the present invention, Preamble-2 may beimplemented using code division for multiple antenna mapping. FIG. 18shows the examples of using Walsh code for the subcarriers and mapped onthe antennas. FIG. 18(a) shows an example of Walsh code and antennamapping in FUSC mode. In this example, Walsh code is mapped ontoantennas 1802. The code used for antenna may be smaller than the codelength, hence the code selection may be considered jointly with PAPRminimization. The Walsh chip is mapped onto subcarriers 1804. The codelength span may be smaller than the coherent bandwidth. The Walsh codemay be covered by another common. Walsh sequence or a PN sequence toallow whitening of other sectors and minimization of cell interference.The code selection may be considered jointly with PAPR minimization.Each cell or sector may be allocated different Walsh/PN sequence forcovering.

FIG. 18(b) shows an example of Walsh code and antenna mapping in PUSCmode, similar to the mappings in FIG. 18(a). FIG. 1(c) shows an exampleof Walsh chip mapping onto subcarriers in time direction. FIG. 18(d)shows an example of Walsh chip mapping onto subcarriers in both time andfrequency domain.

FIG. 19 shows examples where Steiner approach for cyclic shift in timedomain is used to construct Preamble-2 for a low PAPR and low crosscorrelation sequence in FUSC mode (FIG. 19(a)) and in PUSC mode (FIG.19(b)).

FIG. 20(a) shows an embodiment of an FFT size 1024 hybrid transmissionof Preamble-2 in FUSC mode is illustrated, the subcarriers of Preamble-2are partitioned into four groups 2002-2008, each antenna modulates thesubcarriers in the corresponding group. Auto-correlation based on therepetition structure time is used. The presence of the null carriersallows interference avoidance and CIR estimation, and also time domainrepetition structure to support auto-correlation based coarsesynchronization.

FIG. 20(b) is a corresponding hybrid transmission of Preamble-2 in FUSCmode in FFT size 512 (2010); FFT size 256 (2012) and FFT size 128(2014), respectively.

FIG. 20(c) shows an example of relation between the antenna and segmentmapping for PUSC.

Table 2 is a summary of the length of Preamble-2 sequence, the number ofthe OFDM symbols varies with different FFT sizes as well as with thenumber of antennas. For example, while in FUSC mode with two antennas,for FFT size of 256, the length of the common sequence is 64 while forFFT size of 2048, the length of the common sequence is 512.

Antenna FFT 128 256 512 1024 2048 1 FUSC 64 128 256 512 1024 1 PUSC ~21~42 ~85 ~170 ~341 2 FUSC 32 64 128 256 512 2 PUSC ~10 ~21 ~32 ~64 ~128 3FUSC ~10 ~21 ~32 ~64 ~128 3 PUSC ~3 ~7 ~10 ~21 ~32 4 FUSC 16 32 64 128256 4 PUSC ~5 ~10 ~21 ~32 ~64

As discussed before, the scalability of the sequence length provides theadvantage of same synchronization performance at different channelbandwidth. However, it is also possible to use same number of OFDMsymbols or different sequence length in different channel bandwidths, atcorresponding varying synchronization performance.

In accordance with embodiments of the present invention, Preamble-1 andPreamble-2 sequence may be real or complex sequence. One of thecharacteristics of the sequence is the lower cross correlation and PAPRvalue. The sequences may be generated using methods known to thoseskilled in the art, for example, computer searched PN sequence; Golaysequence; generalized linear chirp sequence or CAZAR sequence.

FIG. 21 illustrates the operation arrangement at the receiving antenna.

In FIG. 21(a) Preamble-1 and DL_MAP may be searched and demodulated by aplurality of antennas 2102, 2104. In another embodiment as shown in FIG.21(b), Preamble-1 may be searched by single receive antenna 2108, theother receive antennas may be used to search other frequencies. DL_MAPcan be demodulated by single receive antenna 2110, the other receiveantennas may be used to demodulate other BS_DL_MAP. In both embodiments,Preamble-2 is detected by multiple antennas 2106 to ensure robustness.

FIG. 22 is an exemplary flow chart of a method to detect legacy preambleand Preamble-1 at the subscriber station. The mobile subscriber station(MSS) first scans the frequency 2201, then searches legacy preamble2202, if a legacy preamble is found at 2204, the subscriber stationdetects the anchor base station, its IDcell and the segment at 2206. Atstep 2208, the subscriber station detects an information element, forexample, DC_Common_SYNC_JE, based on the information inDC_Common_Sync_IE, the subscriber station searches Preamble 1 2210, thesubscriber station then searches legacy preamble location for the basestations on the active set 2212, subscriber station then detects theIDcell and segment for the other base stations on the active set 2214.

In FIG. 23, the subscriber station first scans the frequency 2301 andthen searches the Preamble-1 first 2302, when it detects a Preamble-12304, it searches the anchor base station using the legacy preamblewithin the limited search window 2306, and detects the anchor basestation's IDcell and segment 2308. The subscriber station then detectsan information element, for example, the DL_Common_Sync_Map_IE in DL_MAP2310, the subscriber station then searches for Preamble-1 2312,subscriber station then searches active base stations at legacy preamblelocations 2314, and detects the IDcell and the segment for the basestations on the active set.

The present invention enables fast cell search which is important tosoft handover and fast cell selection in a mobile broadband wirelessaccess system; supports channel estimation and channel qualitymeasurement for multiple antennas; allows better and simpler initialaccess based on the repetitive structure of the preambles. The presentinvention further provides flexibility by supporting both two symbolsbased preamble and single symbol based preamble; reduces the subcarrierseparation for channel estimation, for example in MIMO systems; andincreases the length of cell specific codes which is important for smallFFT sizes. The present invention further provides non-overlappingallocation of subcarriers to multiple antennas, with no interferencebetween different antennas; supports frequency domain fine frequencysynchronization for two symbols based preamble; supports MIMO andnon-MIMO subscriber stations and allows transmit antenna identification.

What is claimed is:
 1. A subscriber station in an orthogonal frequencydivision multiplexing access (OFDMA) system, the subscriber stationcomprising: receive circuitry adapted to receive and downconvert aplurality of OFDM symbols, wherein the plurality of OFDM symbols form anOFDMA frame having a training sequence, the training sequence includes afirst training sequence and a second training sequence, wherein thefirst training sequence of the training sequence is a first preamblehaving a common synchronization channel comprising commonsynchronization subcarriers, the common synchronization subcarrierscarrying a common sequence and the first training sequence furtherincluding signaling information pertaining to the configuration of thesecond training sequence, wherein the second training sequence is asecond preamble; Fourier Transform (FT) logic adapted to provide a FT oneach of the plurality of OFDM signals to generate a plurality ofdivided-multiplexed coded signals; and decoder logic adapted to providedivision-multiplexing decoding on the plurality of divided-multiplexedcode signals to recover data from a plurality of base stations.
 2. Thesubscriber station according to claim 1, wherein the common sequenceprovides locations of candidate base stations, and reduces a fine searchwindow for the base stations specific preamble.
 3. The subscriberstation according to claim 2, wherein the first training sequence is ata flexible location that is indicated by a preceding zone in the OFDMAframe.
 4. The subscriber station according to claim 2, wherein thesecond training sequence of the training sequence comprisescell-specific synchronization subcarriers, the second preamble beingassigned for use in a base station of the plurality of base stations andperforming cell search at the subscriber station.
 5. The subscriberstation according to claim 2, wherein the common sequence comprisespseudo-noise codes and results in a low peak-to-average power ratio(PAPR).
 6. The subscriber station according to claim 2, wherein theOFDMA system is a multiple-input multiple-output (MIMO) system and thecommon synchronization subcarriers are assigned to a plurality ofantennas according to a predefined antenna mapping pattern.
 7. Thesubscriber station according to claim 6, wherein: the predefined antennamapping pattern is based on time division; a plurality of cell-specificsynchronization subcarriers of the second preamble are divided into aplurality of groups, and each of the plurality of groups of thecell-specific synchronization subcarriers is transmitted at a predefinedtime.
 8. The subscriber station according to claim 1, wherein theplurality of OFDM symbols are downconverted to an intermediate orbaseband frequency signal and then digitized into one or more digitalstreams.
 9. The subscriber station according to claim 8, wherein thedigitized one or more digital streams demodulated, decoded and errorcorrected.
 10. The subscriber station according to claim 2, wherein thecommon synchronization channel is separated by null-subcarriers.
 11. Amethod for operating a subscriber station in an orthogonal frequencydivision multiplexing access (OFDMA) system, the method comprising:receiving and downconverting a plurality of OFDM symbols, wherein theplurality of OFDM symbols from an OFDMA frame having a trainingsequence, the training sequence includes a first training sequence and asecond training sequence, wherein the first training sequence of thetraining sequence is a first preamble having a common synchronizationchannel comprising common synchronization subcarriers, the commonsynchronization subcarriers carrying a common sequence and the firsttraining sequence further including signaling information pertaining tothe configuration of the second training sequence, wherein the secondtraining sequence is a second preamble; transforming each of theplurality of OFDM signals using a Fourier Transform (FT) to generate aplurality of divided-multiplexed coded signals; and decoding theplurality of divided-multiplexed code signals to recover data from aplurality of base stations.
 12. The method according to claim 11,wherein the common sequence provides locations of candidate basestations, and reduces the fine search window for the base stationspecific preamble.
 13. The method according to claim 12, wherein thefirst training sequence is at a flexible location that is indicated by apreceding zone in the OFDMA frame.
 14. The method according to claim 12,wherein the second training sequence of the training sequence comprisescell-specific synchronization subcarriers, the second preamble beingassigned for use in a base station of the plurality of base stations andperforming cell search at the subscriber station.
 15. The methodaccording to claim 12, wherein the common sequence comprisespseudo-noise codes and results in a low peak-to-average power ratio(PAPR).
 16. The method according to claim 12, wherein the OFDMA systemis a multiple-input multiple-output (MIMO) system and the commonsynchronization subcarriers are assigned to a plurality of antennasaccording to a predefined antenna mapping pattern.
 17. The methodaccording to claim 16, wherein: the predefined antenna mapping patternis based on time division; a plurality of cell-specific synchronizationsubcarriers of the second preamble are divided into a plurality ofgroups, and each of the plurality of groups of the cell-specificsynchronization subcarriers is transmitted at a predefined time.
 18. Themethod according to claim 11, wherein the plurality of OFDM symbols aredownconverted to an intermediate or baseband frequency signal and thendigitized into one or more digital streams.
 19. The method according toclaim 18, wherein the digitized one or more digital streams demodulated,decoded and error corrected.
 20. The method according to claim 12,wherein the common synchronization channel is separated bynull-subcarriers.